I agree it looks like a high resistance between the starter relay and the negative side of the battery. The reason there is 11.8V on the black wire is because the actuation coil of the load relief relay uses the starter as it's ground path. The connection is made at the output side of the starter relay, the same location as the black wire you are measuring. The starter resistance, probably around 15 mOhms (0.015 Ohms), is ultimately the mechanism that turns off the headlights and other high load stuff when the starter button is pressed. If there was a good ground path back to the battery there should only be a very small voltage measured at the black wire unless the starter button is pressed. When the high starting current flows through the starter the voltage at that point should rise to several volts (V=IxR). The battery voltage will also drop due to it's internal resistance. This is probably around 9V in normal operation. The load relief relay coil will now not have enough voltage drop across it to keep the relay closed so as long as high current is flowing all the loads tied to the relay are turned off.
The fact that the voltage on the black wire rises to 12.6V when the starter button is pressed indicates that the starter relay seems to be working. If you want to be adventurous you can try grounding the black wire on the starter relay to see if the lights come on. WARNING, DO NOT PRESS THE STARTER BUTTON IF YOU TRY THIS. If you do you will let all the smoke out of the jumper wire
. If the lights come on then this is just more support for the theory that a high resistance is present in the starter circuit.
There are only a few candidates I can think of that could be causing a problem like this.
- The starter relay terminal connection.
- The 6mm black wire from there to the terminal behind the battery where a 10mm
black/yellow wire from the reverser control module ties in.
- The terminal connections.
- The 10mm black wire from the terminal to the starter.
- The chassis connection and wire to the negative side of the battery. This is likely
to another terminal connection where many ground returns come back to the
- The starter itself and it's structural mounting connection back to chassis ground.
The battery terminal connection is probably not an issue because other relays seem to be working. If there were a high resistance at either battery terminal then nothing would be working.
The curious thing is that there is any voltage change at the black wire. This would indicate that the path back through the starter is not a complete open circuit. I calculate that the resistance would be around 4000 Ohms. This is based on the assumption that the load relief relay coil is about 250 Ohms and there is the measured drop of 0.8 V at the output of the starter relay. That means 0.8 V divided by 250 Ohms results in about 3 mA (0.003 Amps) (I=V/R) flowing through the relay coil and subsequently the whole starter circuit. 12.6V divided by 0.003 Amps results in 4200 Ohms (R=V/I) in the overall circuit and 250 ohms of that is due to the relay coil.
When the starter relay closes the resistance back to the positive side of the battery is very small. The voltage at the output of the starter relay then rises to close to what the battery voltage is.